1. Field of the Invention
The present invention relates to a positive electrode active material for a lithium secondary battery, a positive electrode for a lithium secondary battery, and a lithium secondary battery.
2. Description of Related Art
In an effort to improve the performance of a lithium secondary battery in which lithium ions are used as charge carriers, it is required to further increase energy density. In order to satisfy such requirements, the development of a positive electrode active material (high-potential positive electrode active material) having a high action potential has progressed. Examples of such a positive electrode active material include a nickel-and-manganese-containing composite oxide (hereinafter, also referred to as “Ni—Mn spinel-structure oxide”). The Ni—Mn spinel-structure oxide exhibits an action potential of 4.3 V or higher (preferably, an action potential of 4.5 V or higher) vs. lithium metal and is a lithium transition metal composite oxide having a spinel crystal structure. For example, Japanese Patent No. 3634694 and Japanese Patent Application Publication No. 2003-197194 (JP 2003-197194 A) discloses examples of the Ni—Mn spinel-structure oxide. In addition, Japanese Patent Application Publication No. 2001-250549 (JP 2001-250549 A) discloses a lithium-containing manganese layered composite oxide in which a part of oxygen atoms (O) is substituted with fluorine atoms (F). In addition, Materials Research Bulletin, 2008, Vol. 43, Issue 12, pp. 3607-3613 discloses a Ni—Mn spinel-structure oxide in which a part of O is substituted with F.
When the Ni—Mn spinel-structure oxide is used as a high-potential positive electrode active material for a lithium secondary battery to further improve performance, one of the objects is, for example, improvement of durability during the use of a lithium secondary battery at a high potential. For example, in a case where this Ni—Mn spinel-structure oxide is used as a high-potential positive electrode active material, when a battery is repeatedly charged and discharged under a condition of being charged such that a positive electrode has a high potential of 4.3 V or higher vs. lithium metal, the battery capacity tends to decrease (deteriorate) along with an increase in the number of times of charging and discharging. One of the reasons is considered to be that, in the high-potential charging state, a transition metal element (for example, manganese) contained in the Ni—Mn spinel-structure oxide is likely to be eluted. In addition, in the high-potential charging state, a nonaqueous electrolyte (typically, a nonaqueous electrolytic solution) is decomposed to produce an additional acid (for example, hydrogen fluoride (HF)). Thus, the elution of a transition metal element from the Ni—Mn spinel-structure oxide may be promoted due to the additional acid. Further, when the temperature of a battery increases (for example, 60° C. or higher) by repeated high-potential charging and discharging, the amount of lithium (Li) deactivated on the surface of a negative electrode may increase. The deviated lithium is unavailable for charge and discharge. As a result, the amount of Li in a positive electrode active material may decrease to cause a deterioration in capacity (that is, a decrease in cycle characteristics).